Fancy swapping an old £5 note for £17,000? Fear not: this is not a scam.

The truth is that some banknotes are collectors' items - and you could be in line for a windfall if you have a valuable one stashed in a drawer.

To raise a large sum, the note has to be rare, of course. The Bank of England has always donated some notes with significant or low serial numbers - indicating one of the first issued in a new series - to people or institutions that were involved in the development of the note.

In May, auctioneer firm Spink & Son auctioned a £5 note from 1971 with the serial number A01 000004 - meaning it was given to then-Chancellor Anthony Barber. It sold for a whopping £16,800.

And there are a number of royals and politicians who traditionally receive a note when a new series is issued.

For example, the Queen receives the first in the sequence, Prince Philip the second and the Prime Minister the third. The fourth goes to the Chancellor of the Exchequer.

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HOW THIS IS MONEY CAN HELP

In May, auctioneer firm Spink & Son auctioned a £5 note from 1971 with the serial number A01 000004 - meaning it was given to then-Chancellor Anthony Barber. It sold for a whopping £16,800.

Pam West, founder of dealer British Notes and author of English Paper Money, says: 'Most commonly, people come across cash in attics where long-gone family members stashed money in hidden compartments of trunks or jewellery boxes.

'Or money turns up under floorboards and in old storage heaters, as many people simply didn't trust banks to look after their money.

There are a number of withdrawn £1, £5, £10, £20 - and even £50 - notes that are worth more than you'd expect. All withdrawn Bank of England notes can be swapped at face value with no time limits

'One customer found a wad of old notes dating back to 1914 in a tweed jacket she bought in a second-hand shop.

'We reckon the gamekeeper who owned the jacket might have been tipped on a drive one day, and forgotten about the money. It turned out to be worth thousands.'

Often, a note's value is closely linked to the serial number (the last six digits) or the prefix (the first four characters). Older notes had a three-character prefix.

Ideally, notes should be in mint condition with no folds, marks or tears on them. The most valuable are those that look like they have just come out of the vault

Andrew Pattison of auctioneer firm Spink & Son

Andrew Pattison, of Spink & Son, says: 'In terms of the serial number, anything at 000999 and under is worth valuing.

'The notes bearing the first or last prefix are valuable, so you're looking for those that start with A or Z.

'But there are others with random letters that are valuable, too, so it's always worth checking with an expert.'

Notes that have notable serial numbers, for example a block of one digit such as 888888, or even 123456, are also snapped up by collectors.

A set of three notes is currently listed on eBay for £7,888. One bears the solid '888888' serial number, and the others are AA01 000111 and AA01 168168. Those with mistakes are valuable, too.

'Some notes are circulated without serial numbers, with a signature missing or, in some cases, we have seen banknotes without the Queen's head,' says Mr Pattison.

'Often, these notes look well-used, which means rather than spotting the potential value, people have been trying to spend them, possibly worrying that they're fakes.'

Pam West, founder of dealer British Notes and author of English Paper Money, says sometimes 'money turns up under floorboards, as many people simply didn't trust banks'

Spink & Son estimates a note with the Queen's head missing could be worth up to £400. We won't all be so lucky to find a stash of notes that will pay for a holiday to the Maldives.

But there are a number of withdrawn £1, £5, £10, £20 - and even £50 - notes that are worth more than you'd expect.

All withdrawn Bank of England notes can be swapped at face value with no time limits, either in person in London or by post.

Alternatively, you can sell notes online using auction site eBay and get better rates.

For example, a £1 note would be swapped at the Bank of England for a £1 coin.

But by doing this, you technically lose out on £1.59.

The £1 note was withdrawn in March 1988. Factoring in inflation, a £1 note would be worth around £2.59 today. By comparison, eBay has £1 notes listed for £2 each.

Older notes from pre-decimalisation are more in demand. A ten-shilling note can be exchanged at the Bank of England for 50p.

Yet they are selling for between £2 and £5 each on eBay. So if you found a wad of 20, you could make £100.

In the money: A note in mint condition bearing the right prefix can fetch thousands on sites such as eBay

Some collectors also prize sets of banknotes with sequential serial numbers. Six ten-shilling notes dated between 1966 and 1970 in 'very good condition' and with consecutive serial numbers sold last week on eBay for £17.10, at around £2.85 each.

Mr Pattison says you should look for notes from before 1960.

'Ideally, notes should be in mint condition with no folds, marks or tears on them,' he says.

'The most valuable are those that look like they have just come out of the vault.'

Plenty of people seem willing to buy new notes, too - such as the polymer £5 note, which came into circulation in September last year.

A note in mint condition bearing the prefix AA01 is currently listed on eBay for £3,900.

One way of making a mint is to stash away your notes for young relatives.

The new £10 note will be issued on September 14, so keep an eye out for low prefixes and serial numbers.

Mr Pattison says: 'If you want to keep a banknote for your grandchildren, or even your great-grandchildren, take a crisp note straight from the bank and place it in a sealable sandwich bag to be stored somewhere dry and flat.'

There are a small number of banknote specialists in the country. You can email a photograph of what you find to dealers or travel there if nearby.

Alternatively, you can attend the World Paper Money Fair on September 29/30 at London's Bloomsbury Doyle Hotel.